The present invention relates to a spot killer circuit for a cathode ray tube (CRT) for removing persistence generated by a remaining beam current, and more particularly, to a spot killer circuit for a CRT which is capable of removing a persistence by controlling an electric potential between a first grid and a cathode in the CRT and preventing damage of the fluorescent body due to inaccurate horizontal and vertical deflection.
A CRT is generally used as a means for displaying a video signal. The CRT includes a first grid G1 for controlling a thermal electron beam, a second grid G2 for accelerating the thermal electron beam, and third and fourth grids G3 and G4 for collecting the accelerated thermal electron beam. The fourth grid G4 functions as an anode. A high voltage of 25 kV to 32 kV applied to the anode is conducted through an internal conductive film of the CRT. Under this configuration, when a beam current which flows from the anode to a cathode remains behind, a phenomenon of leaving a spot on a screen occurs.
That is, when a power shut-off operation is performed frequently in a video apparatus such as a television set, a spot (or "afterimage") is generated on the screen due to a remaining beam current. To solve this problem, a conventional spot killer circuit for a CRT, as shown in FIG. 1, is connected to a first grid G1. By using such a circuit, the spot could be removed to some degree.
FIG. 1 is a circuit diagram showing a conventional spot killer circuit. As shown in FIG. 1, the conventional spot killer circuit is composed of a first power controller 1 for supplying or cutting off a standby voltage under control of a microcomputer, and a second power controller 2 for supplying or cutting off the standby voltage according to whether a power cord is plugged in or not.
The second power controller 2 includes a bias resistor R3 whose one end is connected to the standby voltage, a bias resistor R4 whose one end is grounded, and a transistor Q2 having a base which receives the standby voltage divided by the series bias resistors R3 and R4. The collector of Q2 receives the standby voltage via a resistor R5 and an emitter which is grounded. In the second power controller 2, the collector of the transistor Q2 becomes low when a power cord is plugged in so as to provide the CRT with the standby voltage. Accordingly, the magnitude of second current I.sub.2 flowing through diode D2 is zero. Meanwhile, if the power cord is unplugged, the standby voltage is not provided to the transistor Q2. As a consequence, the electric potential applied to the base of the transistor Q2 becomes gradually lowered. Then, when the electric potential of the base of the transistor Q2 falls below approximate 0.7 V, the operating region of the transistor Q2 is changed into the cutoff region. As a result, the second current I.sub.2 flowing through the resistor R5 charges a capacitor C1 via a diode D2. The voltage of capacitor C1 is applied to the base of a transistor Q3 via a resistor R6. The collector of the transistor Q3 receives a driving voltage via a resistor R7. The collector of the transistor Q3 is connected to a resistor R8 connected in parallel with a capacitor C2. A first grid G1 is connected to the output of the transistor Q3 via the resistor R8. Thus, when the transistor Q3 is in the cutoff region, a third current I.sub.3 charges the capacitor C2.
However, if the second current I.sub.2 flows to charge capacitor C1, the potential of the base of the transistor Q3 raises to switch the operating region of the transistor Q3 into the saturation region, and the third current I.sub.3 is not supplied to the capacitor C2. The electric potential of the collector of the transistor Q3, which receives the driving voltage of about 200 V via the resistor R7, drops to about 0 V immediately after the operating region is switched into the saturation region. Here, the driving voltage is for driving the CRT. Accordingly, the charging potential of the capacitor C2 whose one end is connected to the collector of the transistor Q3 becomes changed. That is, if the capacitor C2 maintains a charged potential of 200 V between its two ends (charge-up occurs when Q3 is in a cut-off state) and then the transistor Q3 switches to the saturation state, the positive end of the capacitor C2 (connected to the collector of Q3) drops down to 0 V and the negative end thereof drops down to -200 V. Accordingly, the bias voltage applied to the first grid G1 which is connected with the capacitor C2 falls down to -200 V. As a result, the operating region of the first grid G1 is switched into the cutoff region to cut off the beam current to thereby remove any spot.
When a user turns on or off a video apparatus using a key input unit (not shown), the first power controller 1 operates to remove the afterimage spot. This will be described below. As an example, if the video apparatus is turned off by means of the key input unit, the microcomputer (not shown) outputs a control signal of a low state. The transistor Q1 which receives the control signal via the resistor R1 is switched into the cut-off region. At this time, the collector of the transistor Q1 becomes high and thus the first current I.sub.1 flows through the diode D1 to charge up the capacitor C1 and provide a bias voltage for Q3 via the resistor R6. Thus, the transistor Q3 is switched into the saturation region. Thereafter, since the bias voltage applied to the first grid G1 falls down to -200 V, the operating region of the first grid G1 is switched into the cutoff region. Thus, the spot is removed by cutting off the beam current as described above.
In other words, the conventional afterimage removal circuit for the CRT lowers the bias voltage applied to the first grid G1 down to -200 V under control of the microcomputer when the video apparatus is turned off, or when the power cord is unplugged, to thereby suppressing generation of the beam current to kill the afterimage spot.
However, although the electric potential of the first grid G1 is lowered down to -200 V, the electric potential of the cathode maintains about 0 V. As a result, a spot phenomenon displayed brightly on a part of the screen can still occur. That is, if the beam current is not completely cut off, the spot is left on the screen.
Further, when the operation of deflecting the thermal electron beam horizontally or vertically is inaccurate, the fluorescent body of the CRT may be damaged.